Drive circuit for an oscillator

ABSTRACT

The present invention concerns a drive circuit for driving an oscillator. The drive circuit comprises a first inductor comprising a first terminal and a second terminal; an electrical energy source connected to the first terminal; and a switching circuit connected to the second terminal and to the oscillator. The switching circuit is configured to operate at least in an off state, where it is configured not to feed electrical energy to the oscillator, and in an on state, where it is configured to feed electrical energy to the oscillator. The first inductor is arranged to store energy in its magnetic field when the switching circuit is in the off state, and, when the switching circuit is in the on state, the switching circuit is arranged to use at least some of the energy stored in the magnetic field to deliver a surge of current from the electrical energy source to the oscillator.

This application claims priority from European Patent application15189081.1 of Oct. 9, 2015, the entire disclosure of which is herebyincorporated herein by reference.

TECHNICAL FIELD

The invention relates to the field of drive circuits for driving anoscillator circuit, such as a crystal oscillator used in watches, forexample. The invention also relates to an electronic circuit comprisingthe drive circuit and the oscillator and to a method for driving theoscillator.

BACKGROUND OF THE INVENTION

The circuit diagram of FIG. 1 illustrates an electronic circuit 1comprising a circuit 3 representing a crystal oscillator and a prior artdrive circuit 5 for driving the crystal oscillator 3. As can be seen,the crystal oscillator is modelled by a first capacitor C₁, a secondcapacitor C₂, an oscillator inductor L_(O), a resistor R_(O), a thirdcapacitor C₃ and a fourth capacitor C₄ connected in parallel with thesecond capacitor, the coil and the resistor. One of the electrodes ofthe first and third capacitors is connected to ground. In the circuit ofFIG. 1, the drive circuit is modelled by an inverter circuit 7. Thiskind of drive circuit is currently widely used to drive crystaloscillators, for example. The inverter circuit 7 may comprise at leastan n-type metal-oxide-semiconductor (MOS) transistor and a p-type MOStransistor, for example. The crystal oscillator and the drive circuitare connected so as to form a positive feedback circuit, which inducesan oscillation.

The signal diagram of FIG. 2 illustrates the behaviour of the signals ofthe electronic circuit 1 of FIG. 1. It is to be noted that, to betterillustrate the results, the resistor R_(O) has been omitted. The first(top) graph illustrates the voltage of the second capacitor C₂ as afunction of time. In other words this graph illustrates the oscillationof the crystal oscillator. The second graph illustrates the outputcurrent of the drive circuit over time, while the third graph shows thedrive circuit output voltage. The drive circuit 5 of FIG. 1 drives thecrystal oscillator 3 for a full half-cycle of the oscillation. In thesecond graph in FIG. 2, it can be seen that the current from the drivecircuit flows in both directions during a semi-period of oscillation,which means that the energy in the oscillator inductor L_(O) of thecrystal oscillator 3 is not increased and the voltage across the crystaloscillator does not increase. Assuming ideal conditions, i.e. noresistance and every component ideal, then the current provided by thedrive circuit 5 must be dissipated in the drive circuit 5. In a realapplication this means that the energy is dissipated in the parasiticresistance of the transistors of the drive circuit 5.

Thus it becomes clear that the electronic circuit illustrated in FIG. 1is not optimal in terms of power consumption. More specifically, thecurrent flow from the driver 5 is not in phase with the current flow ofthe crystal oscillator 3, and this causes power dissipation in thetransistors of the driver 5. A further problem may arise due to thebrief but significant current which may flow in the inverter 7 duringeach switching transition of the inverter 7. Such a transition currentspike may arise if the inverter comprises an NMOS-PMOS transistor pairas mentioned above, where this pair effectively presents a short-circuitwhen both are momentarily on.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problemsidentified above related to oscillator drive circuits.

According to a first aspect of the invention, there is provided a drivecircuit for driving an oscillator, the drive circuit comprising:

a first inductor comprising a first terminal and a second terminal;

an electrical energy source connected to the first terminal; and

a switching circuit connected between the second terminal and theoscillator, the switching circuit being configured to operate at leastin an off state, in which it conveys substantially no electrical energyto the oscillator, and in an on state, in which it conveys electricalenergy to the oscillator, said switching circuit including a firstswitch and a second switch,

wherein the first inductor is arranged to store energy in its magneticfield while the switching circuit is in the off state, and, when theswitching circuit is in the on state, the switching circuit is arrangedto use at least some of the energy stored in the magnetic field todeliver a surge of current from the electrical energy source to theoscillator,

wherein the first switch is a first NMOS transistor and the secondswitch is a second PMOS transistor, and

wherein gate terminals of the first and second MOS transistors areconnected to a common node, a drain of the first NMOS transistor and asource of the second PMOS transistor being connected to the secondterminal of the first inductor, and wherein the switching circuit isconfigured to be in the off state when the electrical potential at thecommon node is greater than a first threshold value, and in the on statewhen an electrical difference potential between the second terminal andthe common node is above a second threshold value.

The proposed new solution has the advantage that the operation of thedrive circuit is more energy efficient than the state of the art drivecircuits for oscillators. This is achieved by avoiding short circuitcurrent and by driving the oscillator in phase with the oscillations ofthe oscillator, as explained later more in detail.

According to a second aspect of the invention, there is provided anelectronic circuit comprising the drive circuit and an oscillator drivenby the drive circuit.

According to a third aspect of the invention, there is provided a methodof driving the oscillator.

Other aspects of the invention are recited in the dependent claimsattached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description of a non-limiting exemplary embodiment, withreference to the appended drawings, in which:

FIG. 1 is a circuit diagram illustrating a state of the art electroniccircuit comprising a crystal oscillator and a drive circuit for drivingthe crystal oscillator;

FIG. 2 is a signal diagram illustrating the behaviour of the signals inthe electronic circuit of FIG. 1;

FIG. 3 is a circuit diagram illustrating an electronic circuitcomprising a crystal oscillator and a drive circuit according to anembodiment of the present invention for driving the crystal oscillator;and

FIG. 4 is a signal diagram illustrating the behaviour of the signals inthe electronic circuit of FIG. 3.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An embodiment of the present invention will now be described in detailwith reference to the attached figures. The invention will be describedin the context of a drive circuit for driving a crystal oscillator alsoknown as a quartz of a watch. However, the disclosed drive circuit isnot limited to be used in the context of crystal oscillators. Identicalor corresponding functional and structural elements which appear indifferent drawings are assigned to the same reference numerals.

FIG. 3 illustrates an electronic circuit 9 comprising a crystaloscillator 3, or more precisely a circuit representing the crystaloscillator 3, as in FIG. 1. However, in the crystal oscillator of FIG.3, the resistor R_(O) has been omitted. As can be seen, the drivecircuit 11 according to this embodiment differs from the drive circuit 5of FIG. 1. The drive circuit 11 comprises an electrical energy source13, in this example a constant voltage source 13, connected betweenground and a first terminal of a drive inductor L_(D), also referred toas a first inductor. A second terminal of the drive inductor isconnected to a switching circuit, which is also connected to the crystaloscillator 3 as shown in FIG. 3. The switching circuit in this examplecomprises a first switch 17 and a second switch 19. In this illustratedexample, the first switch is an NMOS transistor 17, while the secondswitch is a PMOS transistor 19.

As can be seen in FIG. 3, the drain terminal of the NMOS transistor 17is connected to the second terminal of the drive inductor L_(D), whilethe source terminal of the NMOS transistor 17 is connected to ground.The gate of the NMOS transistor is connected to the crystal oscillator3, and more particularly to a first node 21 located between theoscillator inductor L_(O), also referred to as a second inductor, andthe second capacitor C₂. The source of the PMOS transistor is connectedto the second terminal of the drive inductor L_(D) while the drain ofthe PMOS transistor 19 is connected to the crystal oscillator 3 and morespecifically to a second node 23 located between the oscillator inductorL_(O) and the third capacitor C₃. The gate of the PMOS transistor isconnected to the first node 21.

In the present description, the NMOS transistor 17 is on (i.e. thechannel from drain to source is conductive), when its gate-sourcevoltage is positive, and more specifically with its gate voltage above afirst threshold value, such as 0.5 V from the ground. In the presentdescription, the PMOS transistor 19 on the other hand is on (i.e. thechannel from source to drain is conductive) when its source-gate voltageis close to or greater than a second threshold of about 0.6 V. In thisparticular case, the second threshold value is greater than the firstthreshold value. This means that during a short time period the NMOS andPMOS transistors may both be conductive.

Next, the operation of the electronic circuit 9 is explained in moredetail. In an initial state, when no electrical energy is supplied tothe crystal oscillator 3, the capacitors of the crystal oscillator 3 aredischarged and no current flows in the crystal oscillator. At this stagethe gate voltages of the NMOS and PMOS transistors 17, 19 are 0 V andonly the PMOS transistor is conductive. In other words the switchingcircuit is in an on state. When the voltage source 13 is turned on,current starts to flow through the drive inductor L_(D) and through theconductive source-drain channel of the PMOS transistor 19 to the crystaloscillator 3. At this point, current starts to flow in the crystaloscillator 3, and the capacitors C₁, C₂, C₃, C₄ start to charge. At thesame time, the gate voltages of the NMOS and PMOS transistors 17, 19start to rise. This also means that the PMOS transistor 19 turns off asthe NMOS transistor 17 turns on, with the result that the current nolonger flows to the crystal oscillator 3, but flows instead through theconductive drain-source channel of the NMOS transistor 17 to ground. Inthis state, the switching circuit is in an off state. When the NMOStransistor 17 is on, the current flowing through the drive inductorL_(D) stores energy temporarily in a magnetic field, for example in itscoil. The drive inductor L_(D) is therefore in a charging or energisingstate.

The crystal oscillator has thus now started to oscillate, thanks to theenergy provided to it at the beginning. This means that NMOS transistor17 turns off again, while the PMOS transistor 19 turns on, and themagnetic field energy is delivered as additional current flowing throughthe PMOS transistor 19 to the second node 23 in the crystal oscillator 3to charge the capacitors in the crystal oscillator. In other words, atthis point, the energy stored in the magnetic field “pushes” currentfrom the voltage source 13 to the crystal oscillator 3. Now, the driveinductor L_(D) is therefore in a discharging or de-energising state. ThePMOS transistor 19 then turns off again, as the NMOS transistor 17 turnson. The operation explained above will then be repeated according to agiven sequence while the crystal oscillator 3 continues to oscillate.

The signal diagram of FIG. 4 illustrates the behaviour of the signals inthe electronic circuit of FIG. 3. The two vertical lines in the diagramof FIG. 4 show the duration of a full signal cycle. The first (top)graph shows the variation of the current through the drive inductorL_(D). The second graph from the top illustrates the variation of thecrystal oscillator voltage, i.e. the electrical potential at the firstnode 21. The electrical potential measured at this node substantiallycorresponds also to the electrical potential at the gates of the firstand second transistors 17, 19. The third graph from the top shows thecurrent flow in the crystal oscillator 3, i.e. the magnitude of thecurrent flowing through the oscillator inductor L_(O). Finally, the lastgraph illustrates the voltage of the crystal oscillator, i.e. thevoltage across the third capacitor C₃. It is to be noted that thenumerical values in FIG. 4 are merely exemplary and ignore theresistance in the crystal oscillator 3. The voltage maintained by theconstant voltage source 13 is about 3 V in this example.

In view of the explanation above, the operation of the NMOS and PMOStransistors can be approximated by stating that the NMOS transistor 17is on and the PMOS transistor 19 is off during a positive half-cycle ofthe crystal oscillator voltage (second graph from the top in FIG. 4);whereas the NMOS transistor 17 is off and the PMOS transistor 19 is onduring a negative half-cycle of the crystal oscillator voltage. Thus,during a positive half-cycle of the crystal oscillator voltage, themagnetic field energy increases, and when the crystal oscillator voltagebecomes negative (second graph in FIG. 4), the charge is pushed by themagnetic field of the drive inductor L_(D) into the third capacitor C₃(last graph in FIG. 4), thereby helping the oscillator inductor L_(O) toincrease its voltage again (second graph in FIG. 4).

As has been explained above, according to the present invention energyis temporarily stored in the drive inductor L_(D). In this way, theshort circuit current which occurs in the prior art drive circuits canbe avoided, because the energy is stored in the drive inductor L_(D)instead of being shorted to ground. Furthermore, the electrical energyfrom the voltage source 13 is injected to the crystal oscillator 3synchronously (i.e. in phase) with the oscillations of the crystaloscillator, with the help of the energy stored in the drive inductorL_(D). This means that the electrical energy is provided to the crystaloscillator 3 only at given time instants, and more particularly when theelectrical potential at the first node 21 is below the second thresholdvalue. Thus, according to the present invention, the crystal oscillator3 is fed with the current from the drive circuit 11 only when thecrystal oscillator is ready to receive this current. In other words,according to the present invention the drive circuit 11 is in phase withthe crystal oscillator 3 and the crystal oscillator 3 is not drivenagainst the current flow of the crystal oscillator 3. It is to be notedthat the feed inductance, together with the oscillation frequency andthe voltage of the constant voltage source 13 determine the amount ofenergy delivered to the oscillator 3 on each cycle. Thus, the value ofthe feed inductance may need to be tuned to the frequency, or to theparticular oscillator.

The present invention also relates to a method of driving the crystaloscillator 3 by supplying electrical energy from an electrical energysource 13 to the oscillator 3 via the drive inductor L_(D) such that thedrive inductor L_(D) is energised by the electrical energy source 13during a first phase of the oscillator cycle, during which theoscillator 3 is in a non-receptive state, and such that at least some ofthe energy stored in the drive inductor L_(D) during the first phase isused to transfer electrical energy from the electrical energy source 13to the oscillator 3 during a second phase of the oscillator cycle,during which the oscillator is in an energy-receptive state.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive, theinvention being not limited to the disclosed embodiment. Otherembodiments and variants are understood, and can be achieved by thoseskilled in the art when carrying out the claimed invention, based on astudy of the drawings, the disclosure and the appended claims. Forinstance, the number of terminals of the transistors can be more thanthree. The number of terminals in some variants is four. Moreover, thedrive circuit can comprise further circuit elements, such as a capacitorpre-charging circuit for charging the capacitors of the oscillator, acurrent limiter for limiting the currents in the electronic circuit 11and/or a control circuit for controlling the switching of the switches17, 19 of the drive circuit 11 for example.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that different features are recited in mutuallydifferent dependent claims does not indicate that a combination of thesefeatures cannot be advantageously used. Any reference signs in theclaims should not be construed as limiting the scope of the invention.

What is claimed is:
 1. A drive circuit for driving an oscillator, thedrive circuit comprising: a first inductor including a first terminaland a second terminal; an electrical energy source connected to thefirst terminal; and a switching circuit connected between the secondterminal and the oscillator, the switching circuit being configured tooperate at least in an off state, in which the switching circuit conveyssubstantially no electrical energy to the oscillator, and in an onstate, in which the switching circuit conveys electrical energy to theoscillator, said switching circuit including a first switch and a secondswitch, wherein the first inductor is configured to store energy in amagnetic field while the switching circuit is in the off state, and,when the switching circuit is in the on state, the switching circuit isarranged to use at least some of the energy stored in the magnetic fieldto deliver a surge of current from the electrical energy source to theoscillator, wherein the first switch is a NMOS transistor and the secondswitch is a PMOS transistor, and wherein gate terminals of the NMOS andPMOS transistors are connected to a common node, a drain of the NMOStransistor and a source of the PMOS transistor being connected to thesecond terminal of the first inductor, and wherein the switching circuitis configured to be in the off state when electrical potential at thecommon node is greater than a first threshold value, and in the on statewhen an electrical difference potential between the second terminal andthe common node is above a second threshold value, wherein a drain ofthe PMOS transistor is connected to the oscillator.
 2. The drive circuitaccording to claim 1, wherein the second threshold value is higher thanthe first threshold value.
 3. The drive circuit according to claim 1,wherein the first and second threshold values are positive voltages. 4.The drive circuit according to claim 1, wherein the first and secondthreshold values are between 0 V and 1 V.
 5. The drive circuit accordingto claim 1, wherein a source terminal of the first transistor isconnected to ground, while the drain terminal of the NMOS transistor isconnected to the second terminal.
 6. The drive circuit according toclaim 1, wherein the source terminal of the PMOS transistor is connectedto the second terminal.
 7. The drive circuit according to claim 1,wherein the electrical energy source comprises a constant voltagesource.
 8. An electronic circuit comprising the drive circuit accordingto claim 1 and the oscillator.
 9. The electronic circuit according toclaim 8, wherein the oscillator comprises a second inductor comprising athird terminal and a fourth terminal, and a capacitor connected inseries with the second inductor, and wherein the drain terminal of thePMOS transistor is connected to the third terminal.
 10. The electroniccircuit according to claim 9, wherein gate terminals of the NMOS andPMOS transistors are connected to the fourth terminal.
 11. Theelectronic circuit according to claim 8, wherein the operation of theswitching circuit is configured to be controlled by electrical potentialat the fourth terminal.
 12. A method of driving an oscillator by a drivecircuit for driving an oscillator, the drive circuit including a firstinductor including a first terminal and a second terminal, an electricalenergy source connected to the first terminal, and a switching circuitconnected between the second terminal and the oscillator, the switchingcircuit being configured to operate at least in an off state, in whichthe switching circuit conveys substantially no electrical energy to theoscillator, and in an on state, in which the switching circuit conveyselectrical energy to the oscillator, said switching circuit including afirst switch and a second switch, wherein the first inductor isconfigured to store energy in a magnetic field while the switchingcircuit is in the off state, and, when the switching circuit is in theon state, the switching circuit is arranged to use at least some of theenergy stored in the magnetic field to deliver a surge of current fromthe electrical energy source to the oscillator, wherein the first switchis a NMOS transistor and the second switch is a PMOS transistor, andwherein gate terminals of the NMOS and PMOS transistors are connected toa common node, a drain of the NMOS transistor and a source of the PMOStransistor being connected to the second terminal of the first inductor,and wherein the switching circuit is configured to be in the off statewhen electrical potential at the common node is greater than a firstthreshold value, and in the on state when an electrical differencepotential between the second terminal and the common node is above asecond threshold value, wherein a drain of the PMOS transistor isconnected to the oscillator, the method comprising: supplying electricalenergy from an electrical energy source to the oscillator via aninductor such that the inductor is energized by the electrical energysource during a first phase of an oscillator cycle, during which theoscillator is in a non-receptive state, and such that at least some ofthe energy stored in the inductor during the first phase is used todeliver a surge of current from the electrical energy source to theoscillator during a second phase of the oscillator cycle, during whichthe oscillator is in an energy-receptive state.